1. Field of the Invention
The invention relates to the etching of aluminum electrolytic capacitor anode foil of a high cubic texture in a first etching bath containing hydrochloric acid and aluminum chloride using direct current with a high current density and followed by a second step using a similar etching bath and a lower current density to produce a high volt foil that has a significantly higher capacitance than prior art.
2. Description of the Prior Art
In the typical process of etching aluminum foil for use in electrolytic capacitors, an electrolytic bath containing sodium chloride or other salt and sulfate based electrolytes has been used in a first step. Such a solution is typically neutral in pH value. This is frequently followed by a treatment in nitric acid. However, the specific capacitance of the resulting foil does not reach the high values which are achieved with the present invention. Typical CV product values at 260 volts range up to 200 volt-uf/cm.sup.2 with the prior art and up to 368 volt-uf/cm.sup.2 with the invented process.
U.S. Pat. No. 4,213,835 discloses a method for electrolytically etching a recrystalized aluminum foil (high cubicity foil) which allows manufacture of foils with exclusively pure cylindrical or cubical etch tunnel structures and tunnel densities greater than 10.sup.7 /cm.sup.2 of foil surface. This process uses a potentiostatic etching technique. The problem with this technique is that it does not lend itself to large scale mass production of etched foils.
U.S. Pat. No. 4,420,367 discloses a method for etching a recrystallized aluminum foil for electrolytic capacitors by carrying out an electrolytic tunnel etching process in a first etching stage, as is common in the art. The improvement of this patent relates to the provision that the continued etching for enlarging the tunnels is nonelectrolytic, being accomplished chemically in one or several etching stages. In the first stage, the recrystallized aluminum foil is subjected to electrolytic tunnel formation to form a tunnel structure in the aluminum and in the second stage the aluminum foil with tunnel structure from the first etching stage is subjected to at least one further nonelectrolytic etching stage to effect tunnel enlargement by chemical etching.
Still other processes have been used to yield a higher specific capacitance in aluminum foils. The present invention is directed to achieving a significant increase in specific capacitance for high volt foils.